1. Technical Field
The present invention relates to a projection type image display device incorporating a projection optical system adapted to project an image formed by a liquid crystal panel or the like on a screen.
2. Related Art
As a projection optical system adapted to project an image of a projection device disposed in the vicinity of a screen on the screen, there can be cited what has a refracting optical system including a plurality of lenses disposed on a reduction side and a concave reflecting mirror disposed on an magnification side and for folding a light path, and performs enlarged close-range projection on the screen (e.g., JP-A-2004-258620, JP-A-2006-235516).
However, such a projection optical system as described above has an extremely short focal length because of its super wide view angle, and in particular, has a small light beam diameter in the vicinity of the optical axis. In contrast, the light beam towards the periphery of picture plane becomes extremely wide in order for provide an enough amount of peripheral light. Further, in the case in which a cover plate with parallel planes for protecting the concave reflecting mirror is disposed in the vicinity of the concave reflecting mirror, the angle of the light beam entering the peripheral section of the cover plate becomes very superficial due to the wide angle projection, thus considerable deterioration in the image quality and a considerable amount of reflection on the surface of the cover plate are caused. Further, in the projection optical system using this kind of concave mirror, since the light paths are collected near the focal position or the exit pupil position of the concave mirror, it is preferable to dispose the cover plate at a position slightly distant from the light-collected section from the viewpoint of safety and so on. However, in such a case, since the size of the cover plate is apt to increase as the parallel plane cover becomes distant from the light-collected position, and there is caused a problem regarding the strength of the cover plate, supporting method there of, and so on.